A) For the reaction N2(g) + 2O2(g) 2NO2(g):

Here are the equilibrium constant expressions for the given reversible reactions:

*a) For the reaction $\text{N}_2\text{(g)} + 2\text{O}_2\text{(g)} \rightleftharpoons 2\text{NO}_2\text{(g)}$:

$$K_c = \frac{[\text{NO}_2]^2}{[\text{N}_2][\text{O}_2]^2}$$

*b) For the reaction $2\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{N}_2\text{O(g)}$:

$$K_c = \frac{[\text{N}_2\text{O}]^2}{[\text{N}_2]^2[\text{O}_2]}$$

*c) For the reaction $2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{SO}_3\text{(g)}$:

$$K_c = \frac{[\text{SO}_3]^2}{[\text{SO}_2]^2[\text{O}_2]}$$

*d) For the reaction $\text{PCl}_3\text{(g)} + \text{Cl}_2\text{(g)} \rightleftharpoons \text{PCl}_5\text{(g)}$:

$$K_c = \frac{[\text{PCl}_5]}{[\text{PCl}_3][\text{Cl}_2]}$$